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Worksheet Grade 10 Science

Foxglen Reservoir: pH, Alkalinity, and Corrosion-Control Chemistry

A grade 10 chemistry worksheet where students use pH and alkalinity data from a fictional reservoir to reason about corrosion control and drinking water standards.

Objectives

  1. Students will use the logarithmic pH scale to compare the acidity of two or more water samples.
  2. Students will calculate percent change in alkalinity between two water samples.
  3. Students will compare pH and alkalinity readings to real drinking water standards and internal utility targets.

Vocabulary

pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.
Alkalinity
Alkalinity is water's ability to resist becoming acidic, acting like a buffer that keeps pH from swinging wildly when acids are added. Water with good alkalinity helps protect fish and other aquatic life from sudden pH changes.

Also: corrosion, logarithmic scale

Teacher overview

Foxglen Reservoir (a fictional reservoir used for this worksheet) supplies a town whose raw water runs mildly acidic and low in alkalinity before treatment. This worksheet has students read pH and alkalinity test results from Foxglen's plant and distribution system, apply the pH scale's logarithmic structure, and reason about why treatment plants adjust both properties before water reaches customers. A common misconception is treating pH like an ordinary linear scale; because each whole-number drop in pH means a tenfold increase in hydrogen-ion concentration, a pH difference of even one unit represents a large change in acidity, not a small one.

Foxglen Reservoir: pH, Alkalinity, and Corrosion-Control Chemistry — Grade 10 Science
Name: ____________________________ Date: ______________

Student Worksheet

  1. In one or two sentences, define pH and state what it measures.
  2. A sample of treated water leaving Foxglen's plant has a pH of 7.5. A sample from a customer's tap farther along the distribution system has a pH of 6.5. Which sample is more acidic? How can you tell from the pH scale alone?
  3. The pH scale is logarithmic: each whole-number drop of 1.0 pH unit means the hydrogen-ion (H+) concentration increases by a factor of 10. Using the two samples from item 2, is the tap sample's H+ concentration higher or lower than the plant sample's, and by about what factor?
  4. The EPA's secondary (aesthetic) standard for drinking water pH is 6.5 to 8.5. Do both Foxglen samples from item 2 fall within that range? Explain, noting where each sample sits relative to the range's edges.
  5. Foxglen's raw water is naturally soft and mildly acidic, so operators add a chemical to raise its alkalinity before it leaves the plant. In your own words, explain why low-alkalinity, low-pH water is more likely to corrode metal pipes than water with higher alkalinity.
  6. A treated sample leaving Foxglen's plant has an alkalinity of 45 mg/L, measured as calcium carbonate. Suppose a sample taken after traveling through several miles of older pipe has an alkalinity of 38 mg/L. Calculate the percent decrease in alkalinity between the two samples, using Percent decrease = (starting value − ending value) ÷ starting value × 100.
  7. Suppose the utility sets an internal target that treated water leaving the plant must have an alkalinity of at least 40 mg/L to protect pipes from corrosion. Does Foxglen's treated sample from item 6 (45 mg/L) meet that target? By how much does it clear or miss it?
  8. Another sample, taken from a different tap, has a pH of 8.5, the high edge of the secondary standard. Compared to the plant sample's pH of 7.5, is this sample's H+ concentration higher or lower, and by about what factor? Use the same logarithmic rule from item 3.
  9. Explain why testing water at several points across a distribution system, rather than only at the treatment plant, gives a more complete picture of the water quality that actually reaches customers. Use Foxglen's plant-versus-tap pH difference as your example.
  10. Challenge. One more sample has a pH of 5.5, one full unit below the low end of the secondary standard (6.5). About how many times more acidic is this sample than water at pH 6.5? Is this sample within the secondary standard?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: pH measures how acidic or basic (alkaline) a water sample is, based on its concentration of hydrogen ions, on a scale that commonly runs from 0 to 14.
  2. Answer: The tap sample (pH 6.5) is more acidic. Lower pH values indicate greater acidity, and 6.5 is lower than the plant sample's 7.5.
  3. Answer: higher, by about a factor of 10.
    The pH dropped by exactly 1.0 unit (7.5 to 6.5), and each 1.0-unit drop means a tenfold increase in H+ concentration.
  4. Answer: yes, both within 6.5-8.5.
    The plant sample (7.5) sits comfortably inside the range, while the tap sample (6.5) sits right at the low edge.
  5. Answer: Water with low pH and low alkalinity has little capacity to resist chemical reactions with pipe metal, so hydrogen ions in the water more readily dissolve metal from pipe walls; higher alkalinity buffers the water and slows that reaction.
  6. Answer: about 15.6 percent.
    (45 − 38) ÷ 45 × 100 = 7 ÷ 45 × 100 = 15.56, which rounds to 15.6 percent.
  7. Answer: yes, by 5 mg/L.
    45 mg/L is above the 40 mg/L target, clearing it by 45 − 40 = 5 mg/L.
  8. Answer: lower, by about a factor of 10. pH 8.5 is 1.0 unit higher than pH 7.5, and each 1.0-unit rise means the H+ concentration decreases by a factor of 10.
  9. Answer: Water quality can change as it travels through pipes, picking up or losing properties like pH and alkalinity along the way, so a single reading at the plant would miss those changes and could give a false picture of what comes out of a customer's tap.
  10. Answer: about 10 times more acidic. pH 5.5 is 1.0 unit below pH 6.5, so its H+ concentration is about 10 times higher than at pH 6.5. This sample is not within the secondary standard: because 5.5 falls below the 6.5 low edge, it sits outside the 6.5-8.5 range.

Differentiation

Support (below grade level)

Provide a printed number line of the pH scale from 0 to 14 with "more acidic" and "more basic" labeled at each end, and work through item 3's factor-of-10 reasoning together as a class before students attempt items 8 and 10 independently.

Extension (above grade level)

Ask students to calculate the exact factor of change in H+ concentration for the 0.8-unit gap between two additional samples at pH 7.2 and pH 6.4 (hint: the factor equals 10 raised to the power of the pH difference), and explain why this factor is not a whole number the way the worksheet's other factors were.

ELL supports

Pre-teach "acidic," "alkalinity," and "corrosion" with a simple diagram of a metal pipe wearing away next to a labeled pH scale. Allow students to answer items 2, 4, and 7 with short phrases or by circling values on a printed number line rather than writing full sentences.